Air deflectors help mileage, but aren’t vital

04/27/2013 6:29 AM

04/27/2013 6:30 AM

Dear Tom and Ray:

There once was a black plastic thingamajig hanging under the front end of our Mercury Grand Marquis, right behind the front bumper. We could almost always hear it scrape against concrete parking blocks in parking lots. Recently, it was partially torn away by a parking block. Einstein, aka my husband, who knows nothing about cars except how to put gas in the tank and air in the tires, tore away the rest of it. He claims that it is not needed and that we will get better gas mileage without it. I say that the automotive engineers probably put it there for a reason and that it should be replaced. What say you? — Betty

Ray: I’m afraid old Einstein is half right, Betty.

Tom: What broke off is an air deflector. It’s a cheap, plastic device that directs air underneath the car in order to improve mileage.

Ray: So he’s wrong about getting better mileage without it (although maybe you’ll get better mileage than with it hanging halfway off!). But he’s right that you don’t really need the thing.

Tom: A primary principle of aerodynamics is “the smoother, the better.” You want to reduce air turbulence. In terms of shapes that do that, think of an airplane, or a bullet. Because when air passes over something smoothly, it creates less drag than if it’s interrupted by baffles and edges.

Ray: Since there are some hard edges and odd-shaped parts under the bumper and at the front of the lower engine compartment, the manufacturer covered them up with a cheap piece of plastic, to send the air under the car instead of right into that stuff.

Tom: Does it make a big difference in your mileage? No. It’s only a factor at higher speeds. But manufacturers work in fractions of a mile per gallon, knowing that they all add up. So to them, it was worth the small cost. Or maybe it was worth the cost for all the money they’ll make later on replacement air deflectors.

Ray: But is it worth it to you? Probably not. Because this wind deflector is … what’s the nice way to put this? Cheap junk. And because it’s in the direct line of fire of concrete parking blocks, it’s very common for it to break or fall off. Lots of our customers just ask us to tear it all off rather than have to listen to it scraping along the ground.

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